1. Field of the Invention
This invention relates to trimellitic acid anhydride (TMA) and more particularly is concerned with an improved process for the preparation of trimellitic anhydride relatively free from color bodies to obtain a relatively color-less trimellitic anhydride. The invention has particular applicability when the trimellitic anhydride has been produced by a series of reactions and procedures which result in a darkly-colored product, characteristically of a dark brown color.
2. Background
Trimellitic acid (TMLA), the 1,2,4 benzene tricarboxylic acid, is useful as an intermediate in the production of quality plasticizers and polyester resins. For these applications, in which trimellitic acid is esterified with a monohydric or a polyhydric alcohol, the evolution of water as an esterification by-product together with the attendant difficulty of eliminating water from esterification reaction mixtures favors the desirability of employing trimellitic acid as the anhydride rather than as the acid. Resins and plasticizers may further require a trimellitic anhydride which is relatively free from color bodies and also free from the heavy metals employed as catalysts for the air oxidation of aliphatic-substituted benzenes to produce trimellitic acid. A Delta E color of below 2 is often specified for trimellitic anhydride used in white or transparent resins, and a metal content of less than about 50 p.p.m. (parts per million) is desirable to achieve good color and oxidation stability. While the more commonly employed maleic and phthalic acid anhydrides are readily prepared by thermal dehydration of the corresponding acids, and the anhydrides are easily purified by atmospheric pressure sublimation, trimellitic anhydride cannot be processed in this manner. Firstly, the acid requires temperatures in excess of 200.degree. C. for thermal dehydration to take place, and even at these temperatures dehydration is not complete. Secondly, trimellitic anhydride is essentially nonvolatile and must be distilled at temperatures above 250.degree. C. under vacuums on the order of 10-60 mm mercury absolute to prevent color degradation. Also, to increase the ordinarily slow rate of dehydration, it has previously been proposed to employ chemical dehydrating agents such as acetic anhydride, sulfuric acid, phosphorus pentoxide, or the like to dehydrate the last traces of trimellitic acid before distilling the anhydride. These chemical dehydrating agents are costly to recover and regenerate, and consequently impose an expensive operating burden on existing processes for the purification of trimellitic anhydride. Furthermore, their use in some cases results in the substitution of one impurity for another.
Typically, the product resulting from any or all of these procedures can be of a color which is undesirable for product to be used in many applications. It is therefore essential to obtain a product relatively free of color bodies in order to obtain a relatively colorless trimellitic anhydride.
The presence of color bodies is frequently measured by optical scanning equipment, such as available from the Milton Roy Company, Ivyland, Pa. A Milton Roy Color Scan II colorimeter can be used to measure the Delta E color level of trimellitic anhydride. A Delta E color of 1.5 or below is a preferred color level.
Accordingly, an object of the present invention is to provide an improved process for preparing trimellitic anhydride relatively free from color bodies having a Delta E color below 1.5 and a metal content below 50 p.p.m. Other and more particular objects will become apparent as the description of this invention is set forth in detail hereinafter.